In many microbiological procedures, there is a need for separating different types of particles, typically cells, from each other. There are e.g. large scale fluorescence-activated cell sorters, FACS, offering cell throughputs of even up to 20000 particles per second. However, such equipment is complex, expensive and requires well-educated operators. The environment for the particles to be sorted is often also quite tough. In applications when the throughput is not the most crucial parameter, smaller, less complex and less expensive solutions would be to prefer. Such small scale solutions also enable integration with other microfluidic modules to enable several tasks in so called lab-on-a-chip solutions.
There is a number of small scale particle sorter equipments proposed, typically intended as on-chip arrangements. Some techniques rely on physical differences between the particles, so-called label-less sorting. However, since the sorting is dependent on the actual particle properties, such approaches are not generally applicable to all types of particles. One example of a label-less separation method based on acoustic forces on particles in a stationary wave is disclosed in the U.S. Pat. No. 6,929,750. In the published international patent application WO 2004/024287, an apparatus for directing particles in a fluid to a plane surface boundary is disclosed. An ultrasound standing wave with a single pressure node is utilized to move the particles towards a pressure node or anti-node along the standing wave.
A similar technique is utilized in the published U.S. patent application 2006/0163166 to separate particles that are differently responsive to such standing waves.
Another group of techniques are called labelled techniques, where the particles of interest are labelled at some stage. A typical example of such a label is a fluorescent substance selectively attached to the particles of interest. In the stream of particles, the labelling is detected and a successive sorting makes use of the detection information and some switch is used to displace the particles accordingly. The switching can be performed by different techniques, either moving the particles or the fluid which contains the particles. Non-exclusive examples are hydro-dynamical switching, optical sorting, electro-kinetic sorting and magnetic sorting.
Prior art micro-sorting methods have different disadvantages. Some of them expose the particles for incautious handling, e.g. high electrical fields, high optical intensity or heat. Other methods put severe demands on e.g. the fluids in which the particles are transported, e.g. electrokinetic methods, which make integration with other systems difficult. Further other methods involve movable puts, which may be sources for clogging.